1. Field of the Invention
This invention relates to a light emitting device assembly, such as a downward facing light assembly, which is well suited for use with solid state lighting sources, such as light emitting diodes (LEDs).
2. Description of the Related Art
Lighting fixtures are ever-present in residential, commercial and industrial spaces throughout the world. In many instances the lighting fixtures, for example troffer fixtures, are mounted to or suspended from ceilings, or even recessed into the ceiling and house elongated fluorescent light bulbs that span the length of the troffer. For ceiling-recessed troffers, the back side of the troffer extends into the plenum space above the ceiling. The troffer fixture can include elements to dissipate heat generated by the light source into the ambient. Ceiling recessed troffers dissipate heat from the light source into the plenum area above the ceiling where air can be circulated to facilitate the cooling mechanism.
In yet other instances, the down lighting fixtures can be mounted to a pole or canopy in an outdoor setting. The outdoor pole mounted fixtures, such as a floodlight, are broad-beamed high intensity lights that are often used to illuminate an outdoor area. Light fixtures in an outdoor setting are exposed to the environment and must have a weather-proofed housing that protects the light source housed within the housing. Outdoor canopy mounted lighting fixtures are commonly used as a lighting solution for parking garages, parking lots, gas stations and many other outdoor settings. The down light fixtures are typically constructed of aluminum or plastic housing having a box-like shape. The housing also has an opening covered by a clear or translucent cover or lens that transmits light emitted from the light source housed within the housing and a reflector to reflect emitted light towards the cover or lens. Typical light sources used for down lights are neon, fluorescent, metal halide or incandescent lights.
Various types of lighting fixtures are used with different types of light sources, such as incandescent bulbs, fluorescent tubes or metal halide. One of the problems associated with the conventional lighting units is that their light sources can experience relatively low electrical efficiency. In order to provide sufficient lighting, especially in large lighting applications, conventional lights are required to consume a significant amount of energy. For example, a standard fluorescent tube 60 inches in length consumes as much as 60 to 70 Watts, and conventional light fixtures can utilize many of these tubes. Also, typical metal halide fixtures consume as much as 400 Watts.
More recently, with the advent of the efficient solid state lighting sources, these lighting fixtures have been used with LEDs, for example. LEDs are solid state devices that convert electric energy to light and generally comprise one or more active regions of semiconductor material interposed between oppositely doped semiconductor layers. When a bias is applied across the doped layers, holes and electrons are injected into the active region where they recombine to generate light. Light is produced in the active region and emitted from surfaces of the LED.
LEDs have certain characteristics that make them desirable for many lighting applications that were previously the realm of incandescent or fluorescent lights. Incandescent lights are very energy-inefficient light sources with a vast majority of the electricity they consume being released as heat rather than light. Fluorescent light bulbs are more energy efficient than incandescent light bulbs, but are still relatively inefficient. LEDs by contrast, can emit the same luminous flux as incandescent and fluorescent lights using a fraction of the energy.
In addition, LEDs can have a significantly longer operational lifetime. Incandescent light bulbs have relatively short lifetimes, with some having a lifetime in the range of about 750-1,000 hours. Fluorescent bulbs can also have lifetimes longer than incandescent bulbs such as in the range of approximately 10,000-20,000 hours, but provide less desirable color reproduction. In comparison, LEDs can have lifetimes between 50,000 and 70,000 hours. The increased efficiency and extended lifetime of LEDs is attractive to many lighting suppliers and has resulted in LED lights being used in place of conventional lighting in many different applications. It is predicted that further improvements will result in their general acceptance in more and more lighting applications. An increase in the adoption of LEDs in place of incandescent or fluorescent lighting would result in increased lighting efficiency and significant energy saving.